The breed standard for Chis is under 6 lbs. however, many badly bred Chis weight more than this. Whether your girls weight is appropriate should not be based on the general standard but on her body type. You should be able to feel her ribs with little effort, but they should not be visible.
I too have a chi that is super active and has trouble keeping weight on him. What food do you feed her? I've noticed that since I changed my Toby to Ziwipeak his weight has been much better.
At her age, it is normal for her not to be as active as a younger dog. She is at the age when they usually start mellowing and calming down. Totally normal.
As far as heat goes, I am not an expert (I've only had one female dog in my life and she was spayed) but I believe that some Chis can go into "silent heat" where they don't really show any physical signs or bleed. Maybe it is that. Or maybe she just doesn't go into heat often- if she has always been on this schedule, maybe it is normal for her. I would be more worried if she had been going into heat regularly and all of the sudden stopped.
I hope I helped!
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